[InfluxDB] Add nanosecond duration literal support documentation

[jsternberg]

URL for relevant page?

influxdata/influxdb#8406

What products and version are you using?

InfluxDB 1.3.0

If we have somewhere where we mention possible duration literals, the new ns literal should be included there. Previously, it was not possible to specify nanoseconds in places like GROUP BY time(...).

It is still possible to use integers for duration math and the integers will be treated as nanoseconds. So these two are the same: 1d - 10 and 1d - 10ns.

[rkuchan]

Add ns to the Duration units table in the spec and the crosswalk doc:

ns | nanoseconds (1 billionth of a second)